1. Field of the Invention
The present invention relates to a system and method for controlling the air-fuel ratio in an internal combustion engine, wherein the air-fuel ratios for cylinders are switched with predetermined time differences.
2. Description of the Prior Art
There is a conventionally known internal combustion engine described in Japanese Patent Application Laid-open No.295151/1992, in which the air-fuel ratios for cylinders are switched with predetermined time differences.
If the target air-fuel ratio is continuously switched from a rich value (an A point) to a lean value (a B point), as shown in FIG. 27, it is disadvantageous in that the amount of NO.sub.X discharged is suddenly increased at a medium air-fuel ratio, as shown by a solid line. On the other hand, if the target air-fuel ratio is switched in a moment from the rich value (the A point) to the lean value (the B point), it is possible to avoid the sudden increase in amount of NO.sub.X discharged, as shown by a dashed line. However, if the target air-fuel ratio for each of the cylinders is switched in a moment, the engine torque is suddenly varied to generate a shock, thereby bringing about a problem of a degradation in driveability.
Therefore, in the above prior art internal combustion engine, in switching the target air-fuel ratio from the rich level to the lean level, the amounts of fuel injected by fuel injection valves provided at each of the cylinders are sequentially decreased at predetermined intervals beyond a medium air-fuel ratio at which an emission is degraded. This avoids a torque shock which may be otherwise produced when the amounts of fuel injected into all the cylinders are decreased simultaneously, thereby providing an improvement in driveability while preventing the degradation of the emission.
However, the prior art engine suffers from a problem that the prevention of the degradation of the emission and the avoidance of the generation of the torque shock are not reconciled depending upon the operational range of the engine because of the uniformly set time differences for sequentially decreasing the amounts of fuel injected by the fuel injection valves. In addition, there is a problem that only the control of the amounts of fuel injected by the fuel injection valves does not make it possible to completely prevent the torque shock, whereby the degradation of the driveability due to the generation of the torque shock cannot be sufficiently prevented.
When the above prior art internal combustion engine includes an EGR (Exhaust Gas Recirculating) gas circulating system, even if the circulation of an EGR gas is stopped in switching the target air-fuel ratio from the rich to the lean, the amount of the EGR gas in an intake system is not immediately reduced to zero, and the EGR gas remains in the intake system, until a predetermined time has lapsed. If the amount of fuel injected is decreased in this condition to change the air-fuel ratio to the lean side, the following problem is encountered; the combustion of the air-fuel mixture is degraded due to the presence of the EGR gas to bring about a combustion failure such as a misfire.
In the above prior art internal combustion engine, there is a further problem that even if the air-fuel ratio for each of the cylinders is changed to avoid the medium air-fuel ratio at which the amount of NO.sub.x discharged is increased, the exhaust gas from the cylinder with a rich air-fuel ratio is diluted by the exhaust gas from the cylinder with a lean air-fuel ratio, resulting in a reduced purifying efficiency of an exhaust emission control catalyst.